For me:
Step 1 - Carefully tracked expenses over the course of a full year. Researched reasonable rates for things like car repair, home maintenance, health care inflation, and other costs that might not have been included in 1 year's expenses. Over the course of the year, my partner and I discussed priorities, desire to help family, contribute to charity post-FIRE, etc. The end result of this step was an annual spending number that we both felt comfortable with, along with a detailed list of prioritized spending cuts if the SHTF. At the end of the tracking year we were on the same page and had a documented plan that we both understood and were comfortable with.
Step 1a - In parallel to Step 1, I read SWR research - things like the updated Trinity study, BigERN's posts, ran CFireSim and FIRECalc scenarios, etc. to get to a withdrawal rate I felt comfortable with.
Step 2 - Researched Roth Ladder and rule 72(t). I built a plan to get us through the first 5 years, then to 59 1/2, then to 65 (pensions), 70 (SS), and long-term care. If the one big number was good but it didn't support one of those time-frames then the overall plan won't work.
Step 3 - I educated myself on pension and SS rules, funding levels, etc. to see whether or not I wanted to include them in my planning and at what level. Ultimately I decided that if things were so bad that our plan failed without them then it's possible they could fail or be reduced as well, so we decided not to include them even though at 70% they would cover 100% of our planned expenses from 70+ excluding long-term care.
Step 4 - I looked into ACA rules to figure out health care costs in a few scenarios - as-is (with Cost Sharing Reductions), without CSRs, without any subsidies, ACA collapse, etc. I also calculated the taxes we would pay based on each of those scenarios, because need more income each year as health care costs rise.
Step 5 - Used the above information to create an annual and overall number along with a table of my Roth conversion ladder. This included my accessible funds, the Roth ladder contributions, and starting 5 years out the addition of the ladder funding. I did this to ensure that we could make it to 59 1/2.
Step 6 - Calculated what we had in each bucket (accessible funds, 401(k)s, etc.). Looked at where we were short and developed a plan to fill in the shortfalls.
Step 7 - Hit the numbers - 25x annual budget along with sufficient money for the first 5 years until the ladder rungs kick in (Yay!!!).
Step 8 - Funded Donor Advised Fund.
Step 9 - Downshifted to part-time and worked one more low-stress year with tons of vacation time. I don't think this is necessary for everyone, but my job is such that after my certifications expire I won't be able to return to work in my career field. Working OMY, ignoring the pensions and SS, and having a FatFIRE budget with agreed-upon spending cuts should combine to make the plan fairly safe.